This study explores the regulation of human calcium metabolism using Parathyroid Hormone (PTH) and the authentic secretory form of PTH-related protein. The peptide, PTHrP (1,36), when infused into healthy volunteers, can reproduce several features of human hyperparathyroidism (HPT) and humoral hypercalcemia of malignancy (HHM). HPT and HHM differ in ways that remain unexplained but are critical to understand. By infusing PTH and PTHrP continuously over 48 hours, steady-state evaluation of renal calcium reabsorption and renal 1,25 vitamin D production can be accomplished.